1. Field of the Invention
This invention relates to an improved shut-off valve for a sucker rod pumping system used in oil wells. In one aspect, the invention relates to an improved sucker rod pumping system for wells which are periodically subjected to injection of a heated fluid such as steam.
2. Description of the Prior Art
In the thermal stimulation of oil wells, a subterranean, oil-bearing formation in the immediate vicinity of the wellbore is heated to reduce oil viscosity and improve near wellbore permeability by removal of fines and asphaltic deposits. The technique involves basically two steps: (1) an injection phase wherein steam or other hot fluid is injected down the well into the formation, followed by (2) an oil production phase wherein formation fluids are flowed or pumped from the formation up the well. Since the main benefit of steam stimulation is due to reductions in crude viscosity, this technique is normally applied in wells completed in reservoirs producing low API gravity crudes. These wells normally are produced by artificial means such as rod pumping systems.
In rod pump systems, a subsurface reciprocating pump is located near the producing formation and a rod string, referred to as sucker rod string, extends from the subsurface pump to the surface. At the surface the top joint of the rod string, referred to as a polished rod, extends through a stuffing box of the wellhead and attaches to a pumping unit. The pumping unit, which may be a mechanical pumping jack or a hydraulic actuator, imparts reciprocal movement to the rod string which operates the piston or plunger of the subsurface pump.
The stuffing box includes soft packing which provides a dynamic seal for the reciprocating polished rod.
A problem associated with sucker rod pumping systems as applied in wells which are periodically subjected to thermal stimulation, is that the packing in the stuffing box cannot be exposed to the high pressure or high temperature thermal fluids. The conventional stuffing box performs satisfactorily during the production phase of the operation, but during the steam injection phase, the packing is incapable of providing a reliable seal for long periods of time. Most wells subjected to thermal stimulation in the past have been provided with high temperature shut-off valves referred to as blowout preventers. The blowout preventer is placed in the open position during the production phase of the operation. When it is desired to inject fluids down the well, the opposed rams of the blowout preventer are moved to the closed position which seal around the polished rod disposed therein. The blowout preventer, however, is expensive and not fully reliable because of the tendency of the polished rod to become worn or scored during the pumping phase of the operation. Moreover, the soft packing elements of the rams deteriorate in the presence of the hot injection fluids such that they frequently are incapable of establishing and maintaining a pressure seal about the polished rods.